This invention relates to the apparatus and manufacturing method for a fully integrated multicast switch, intended for use in Reconfigurable Optical Add-Drop Multiplexers (ROADMs), for application in reconfigurable optical communications networks.
Optical communications networks have long made use of multiple wavelengths of light, typically in the infrared region, to carry communications signals. Modern Dense Wavelength Division Multiplexing (DWDM) networks make use of 40, 80, or even larger numbers of different wavelengths, to maximize the capacity of individual fibers. Optical Add-Drop Multiplexers (OADMs) are used to “drop” or “add” specific wavelengths at different network nodes or locations. In fixed OADMs, the wavelengths that are dropped or added at a specific network node are determined by the hardware, in particular by the use of fixed optical filters. In order to allow faster reconfiguration of the dropped and added wavelengths, Reconfigurable Optical Add-Drop Multiplexers (ROADMs) are used.
A wide variety of design approaches have been used for ROADMs, with various advantages and disadvantages, in terms of functionality, performance, and cost. In terms of functionality, it is highly desirable for ROADMs to be “colorless”, “directionless”, and “contentionless”. “Colorless” means that any wavelength can be assigned or designated to any of the add or drop ports of the ROADM. “Directionless” means that if the ROADM is connected to multiple other network nodes, then any of the add or drop ports can be routed to (or from) any of the other nodes. The number of directions that a ROADM supports (in other words, the number of other network nodes that it connects to) is often referred to as the “degree” of the ROADM. “Contentionless” is a somewhat newer concept in ROADM design, and refers to the ability of a ROADM to handle the contention that may occur if add or drop traffic that is directed to/from two different network nodes, is using the same wavelength. A contentionless ROADM requires total flexibility in the assigning of wavelengths to specific add or drop ports.
A variety of optical components or modules are used, or have been used, to implement ROADMs, including Wavelength-Selective Switches (WSS), Optical Cross-Connect Switches, Broadcast-and-Select Switches, Tunable Optical Filters, and Multicast Switches, often in combination. These different building blocks of modern ROADMs have different advantages and disadvantages, in terms of providing colorless, directionless, and contentionless (CDC) functionality, and also differ in terms of a variety of performance parameters (including insertion loss as one key parameter) and cost.
FIG. 1 shows a generalized 4-degree ROADM, supporting traffic from/to four directions (or neighboring network nodes), labeled as North, South, East, and West. The ROADM shown in FIG. 1 comprises all of the functional blocks shown in the figure, including the splitters and Wavelength-Selective Switches (WSS) associated with each of the four directions, as well as the Add/Drop banks. Traffic coming from any of the four directions can be directed to any of the other directions (via an optical fiber path from the splitter of the inbound direction, to the WSS of the outbound direction), or it can be directed to the drop side of one or more add/drop banks. (Note that in FIG. 1, only some of the optical fiber paths are shown.) Traffic from the add side of the add/drop bank(s) can be directed to any of the four directions. In this generalized ROADM, the design of the add/drop bank(s) determines whether the ROADM is colorless, directionless, and contentionless.
Multicast switches can be used to implement both the add and drop banks of a ROADM, to implement colorless, directionless, and contentionless functionality. FIG. 2 shows a generalized depiction of a prior art 4×8 multicast switch. More specifically, FIG. 2 shows a multicast switch configured as a drop bank that accepts traffic from four input fibers, IN1 through IN4, (201), representing four degrees, or directions, and directs this traffic to any combination of eight drop ports, labeled as OUT 1 through OUT 8 (202). Since multiple wavelengths may be present on each of the input fibers, tunable optical filters (203) are used at each of the drop ports, to select the specific wavelength that is intended for that drop port. For the purposes of this discussion, the multicast switch includes the four 1×8 splitters (204), and the eight 1×4 (or 4×1) optical switches (205), but does not include the eight associated tunable optical filters (203). One can see that this drop bank design is capable of dropping any wavelength from any one of the four input fibers, at any of the eight drop ports. This includes the ability to direct a given wavelength from one of the inputs, to more than one of the drop ports. This drop bank design is therefore colorless, directionless, and contentionless.
The same 4×8 multicast switch can also be used to implement an add bank that accepts traffic from eight add ports, and directs each of these optical signals to any of the four output fibers. In this case, the tunable filters shown in FIG. 2 are not used, the add ports are the common fibers of the 1×4 optical switches (the fibers shown in FIG. 2 as running between the 1×4 optical switches and the tunable filters), and the optical splitters are providing a multiplexing or combining function, rather than a splitting function. In short, when used as an add bank, the flow of traffic in FIG. 2 is from right to left. Thus, the add ports, labeled as OUT 1 through OUT 8 in FIG. 2, are actually serving as inputs. And the “network side” ports, labeled as IN1 through IN4 in FIG. 2, are actually serving as outputs. Note that traffic from multiple add ports can be directed to the same output fiber. The add bank is therefore colorless and directionless. Whether it is contentionless or not depends on whether the transmitters used at each of the add ports are “tunable” or reconfigurable, to use different wavelengths for their transmitted signal.
In order to provide cost-effective ROADMs that are colorless, directionless, and contentionless, it is important to minimize the cost (and in some cases the physical dimensions) of M×N multicast switches, with M representing the number of input (or output) fibers on the network side, and N representing the number of drop (or add) ports.
In looking at FIG. 2, one can appreciate that as M and/or N increase, the cost, complexity, and size of a multicast switch also increases, where M and N are positive integers, or integers greater than 1. Splitters and 1×N optical switches are commercially available from multiple sources as individual optical components. In many cases, the technology used to implement the splitter function is significantly different from the technology used to implement the optical switching function. It is therefore a typical practice to implement multicast switch modules by integrating or interconnecting individual splitter and optical switch components, using fusion-splicing techniques to connect the fiber pigtails (206) of the separate components. With this approach, the number (and cost) of the splices needed between the individual splitters and switch components increases rapidly, as a function of the product of M and N. For example, a 4×8 multicast switch, as shown in FIG. 2, would require four 1×8 splitter components (204), eight 1×4 optical switches (205), and 32 fusion-splicing operations, with the fusion splicing points labeled as item 207.